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Abstract:

In one aspect, a rechargeable lithium battery including a positive
electrode having a positive active material, a negative electrode having
a negative active material, and an electrolyte is provided. The positive
active material can include manganese-based oxide, and the electrolyte
can include fluoroethylene carbonate, lithium bis(oxalato)borate, and
tris(trialkylsilyl)borate.

Claims:

1. A rechargeable lithium battery comprising a positive electrode
including a positive active material; a negative electrode including a
negative active material; and an electrolyte including an organic
solvent, a lithium salt, and an additive, and wherein the positive active
material comprises manganese-based oxide, and the additive comprises
fluoroethylene carbonate, lithium bis(oxalato)borate, and a
tris(trialkylsilyl)borate.

2. The rechargeable lithium battery of claim 1, wherein the additive
comprises: the fluoroethylene carbonate in an amount of 1 to 20 parts by
weight; the lithium bis(oxalato)borate in an amount of 0.5 to 5 parts by
weight; and the tris(trialkylsilyl)borate in an amount of 0.5 to 3 parts
by weight based on 100 parts by weight of the non-aqueous organic solvent
and the lithium salt.

3. The rechargeable lithium battery of claim 1, wherein the sum of the
lithium bis(oxalato)borate and the tris(trialkylsilyl)borate by parts by
weight is less than the amount of the fluoroethylene carbonate.

14. The rechargeable lithium battery of claim 1, wherein the
tris(trialkylsilyl)borate is a compound represented by Chemical Formula
3: ##STR00006## wherein, R1 to R9 are each a substituted or
unsubstituted C1 to C5 alkyl group.

18. The rechargeable lithium battery of claim 1, wherein the organic
solvent comprises one or more components selected from the group
consisting of ethylene carbonate (EC), ethylmethyl carbonate (EMC), and
dimethyl carbonate (DMC).

20. The rechargeable lithium battery of claim 19, wherein the
tris(trialkylsilyl)borate is tris(trimethylsilyl)borate.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of Korean
Patent Application No. 10-2011-0045338 filed on May 13, 2011 in the
Korean Intellectual Property Office, the disclosure of which is
incorporated herein in its entirety by reference.

BACKGROUND

[0002] 1. Field

[0003] This disclosure relates to a rechargeable lithium battery.

[0004] 2. Description of the Related Technology

[0005] Lithium rechargeable batteries using an organic electrolyte
solution have twice or more the discharge voltage than that of a
conventional battery using an alkali aqueous solution, and accordingly
have high energy density.

[0006] This rechargeable lithium battery operates by injecting an
electrolyte into a battery cell including a positive electrode having a
positive active material that can intercalate and deintercalate lithium
and a negative electrode having a negative active material that can
intercalate and deintercalate lithium.

[0007] The resulting rechargeable lithium battery can be used for a mobile
phone, an electric pocketbook, a watch, and the like. The positive active
material of the rechargeable lithium battery may include lithium metal
oxide and the like, while the negative active material may include metal
lithium and the like. The electrolyte of the rechargeable lithium battery
may be prepared by dissolving a lithium salt in an organic solvent.

[0008] Recently, a rechargeable lithium battery including a
manganese-based active material has been contemplated for devices
requiring high output such as an electric tool and the like. However, the
rechargeable lithium battery including manganese-based active material
may have a problem of increasing resistance during storage at an elevated
temperature.

SUMMARY

[0009] One embodiment of this disclosure provides a rechargeable lithium
battery having decreased resistance when stored at an elevated
temperature and excellent stability at an elevated temperature.

[0010] Another embodiment of this disclosure provides a rechargeable
lithium battery including a positive electrode having a positive active
material; a negative electrode having a negative active material; and an
electrolyte having an organic solvent, a lithium salt, and an additive.
The positive active material may include manganese-based oxide, and the
additive may include fluoroethylene carbonate, lithium
bis(oxalato)borate, and tris(trialkylsilyl)borate.

[0011] The additive may include 1 to 20 parts by weight of the
fluoroethylene carbonate; 0.5 to 5 parts by weight of the lithium
bis(oxalato)borate; and 0.5 to 3 parts by weight of the
tris(trialkylsilyl)borate based on 100 parts by weight of the sum of the
organic solvent and the lithium salt.

[0012] The sum of the parts by weight of the lithium bis(oxalato)borate
and the tris(trialkylsilyl)borate may be less than the amount of the
parts by weight of fluoroethylene carbonate.

[0013] The manganese-based oxide may include a compound represented by the
following Chemical Formula 4.

LiaMnbO4 [Chemical Formula 4]

[0014] In Chemical Formula 4, 0.5≦a≦1.5, and
1≦b≦3.

[0015] The positive active material may further include nickel-based
oxide. The nickel-based oxide may include a compound represented by the
following Chemical Formula 5, a compound represented by the following
Chemical Formula 6, or a combination thereof.

[0018] The positive active material may include 70 to 90 wt % of the
manganese-based oxide and 10 to 30 wt % of the nickel-based oxide.

[0019] Hereinafter, further embodiments will be described in the detailed
description.

[0020] The present embodiments may provide a rechargeable lithium battery
having excellent stability when stored at an elevated temperature due to
resistance decrease.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a schematic view showing a rechargeable lithium battery
according to one embodiment.

[0022]FIG. 2 provides a graph showing LSV (Linear Sweep Voltammetry) of
electrolytes according to Example 1 and Comparative Example 1 when
allowed to stand at an elevated temperature.

[0023]FIG. 3 provides a graph showing initial impedance of each
rechargeable lithium battery of Example 1 and Comparative Example 1 and
its impedance when allowed to stand at an elevated temperature.

DETAILED DESCRIPTION

[0024] Exemplary embodiments will hereinafter be described in detail.
However, these embodiments are only exemplary, and this disclosure is not
limited thereto.

[0025] The rechargeable lithium battery according to one embodiment is
described referring to FIG. 1.

[0026] FIG. 1 is a schematic view showing a rechargeable lithium battery
according to one embodiment.

[0027] FIG. 1 illustrates a rechargeable lithium battery 100, which
includes a negative electrode 112, a positive electrode 114, a separator
113 interposed between the negative electrode 112 and the positive
electrode 114, an electrolyte (not shown) impregnating the separator 113,
a battery case 120, and a sealing member 140 sealing the battery case
120.

[0028] In some embodiments, the electrolyte includes an organic solvent, a
lithium salt, and an additive. According to one embodiment of the present
disclosure, an additive can be added to the non-aqueous organic solvent
with the lithium salt dissolved therein. A rechargeable lithium battery
including an additive, for example, a rechargeable lithium battery
including manganese-based oxide in a positive electrode may have
decreased resistance during the storage at an elevated temperature and
thus, improved storage characteristics at an elevated temperature.

[0029] In some embodiments, the additive may include one or more
components selected from the group consisting of fluoroethylene
carbonate, lithium bis(oxalato)borate, and tris(trialkylsilyl)borate.

[0030] In some embodiments, the fluoroethylene carbonate may be
represented by the following Chemical Formula 1.

##STR00001##

[0031] In some embodiments, the fluoroethylene carbonate with the lithium
bis(oxalato)borate and the tris(trialkylsilyl)borate can be added to an
electrolyte forming a coating layer on the surface of a negative
electrode and thus, decreases resistance, improving charge and discharge
efficiency at an elevated temperature and accordingly, storability at an
elevated temperature.

[0032] In some embodiments, the fluoroethylene carbonate may be included
in an amount of about 1 to 20 parts by weight. In some embodiments, the
fluoroethylene carbonate may be included in an amount of 1 to 10 parts by
weight, or in an amount of 3 to 5 parts by weight based on 100 parts by
weight of the sum of the organic solvent and the lithium salt. In some
embodiments, the fluoroethylene carbonate can be included within the
range of 1 to 10 parts by weight. A rechargeable lithium battery,
including fluoroethylene carbonate, for example, a rechargeable lithium
battery including manganese-based oxide in a positive electrode, may have
decreased resistance during the storage at an elevated temperature and
thus, improved stability at an elevated temperature.

[0033] The lithium bis(oxalato)borate may be represented by the following
Chemical Formula 2.

##STR00002##

[0034] In some embodiments, the lithium bis(oxalato)borate with the
fluoroethylene carbonate and the tris(trialkylsilyl)borate can be added
to an electrolyte forming a coating layer on the surface of a positive
electrode and thus, stabilizes the surface and decreases resistance
thereon. As a result, the electrolyte may improve charge and discharge
efficiency at an elevated temperature and thus, storability at an
elevated temperature.

[0035] In some embodiments, the lithium bis(oxalato)borate may be included
in an amount of about 0.5 to 5 parts by weight. In some embodiments, the
lithium bis(oxalato)borate may be included in an amount of 0.5 to 4 parts
by weight, 0.5 to 2 parts by weight, or 0.5 to 1 parts by weight based on
100 parts by weight of the sum of the organic solvent and the lithium
salt. In some embodiments, the lithium bis(oxalato)borate can be included
within the range of from about 0.5 to 5 parts by weight. In some
embodiments, a rechargeable lithium battery, for example, a rechargeable
lithium battery including manganese-based oxide in a positive electrode,
may have decreased resistance during the storage at an elevated
temperature and thus, improved stability at an elevated temperature.

[0036] In some embodiments, the tris(trialkylsilyl)borate may be
represented by the following Chemical Formula 3.

##STR00003##

[0037] wherein,

[0038] R1 to R9 are each a substituted or unsubstituted C1
to C5 alkyl group.

[0039] In some embodiments, the R1 to R9 in the above Chemical
Formula 3 may be methyl.

[0040] In some embodiments, the tris(trialkylsilyl)borate with the
fluoroethylene carbonate and the lithium bis(oxalato)borate can be added
to an electrolyte forming a coating layer on the surface of a positive
electrode and thus, stabilizes the surface and decrease resistance
thereon. The electrolyte may improve charge and discharge efficiency at
an elevated temperature and thus, storability at an elevated temperature.

[0041] In some embodiments, the tris(trialkylsilyl)borate may be included
in an amount of 0.5 to 3 parts by weight. In some embodiments, the
tris(trialkylsilyl)borate may be included in an amount of 0.5 to 2 parts
by weight, or 0.5 to 1 parts by weight based on the 100 parts by weight
of the sum of the organic solvent and the lithium salt. In some
embodiments, the tris(trialkylsilyl)borate can be included in an amount
of 0.5 to 3 parts by weight in a rechargeable lithium battery, for
example, a rechargeable lithium battery including manganese-based oxide
in a positive electrode, may have decreased resistance at an elevated
temperature and thus, improved storage characteristics at an elevated
temperature.

[0042] In some embodiments, the fluoroethylene carbonate, the lithium
bis(oxalato)borate, and the tris(trialkylsilyl)borate can be used
together to form a coating layer. The coating layer may further decrease
resistance and improve stability at an elevated temperature compared with
when they are independently used or as a combination of only two.

[0043] In some embodiments, the sum of the lithium bis(oxalato)borate and
the tris(trialkylsilyl)borate may be less included than the amount of the
fluoroethylene carbonate. In some embodiments, the three additives can
have a ratio relationship affording a rechargeable lithium battery having
decreased resistance at an elevated temperature and thus, improved
storage characteristics at an elevated temperature.

[0044] In some embodiments, the organic solvent can serve as a medium for
transmitting ions taking part in the electrochemical reaction of the
battery. In some embodiments, the organic solvent may include a
carbonate, ester, ether, ketone, or alcohol moiety. In some embodiments,
the organic solvent may be an aprotic organic solvent.

[0046] The dielectric constant can increase and the viscosity can decrease
when the organic solvent includes a linear carbonate compound and a
cyclic carbonate compound. In some embodiments, the cyclic carbonate
compound and linear carbonate compound can be mixed together in a volume
ratio of about 1:1 to about 1:9.

[0047] Examples of an organic solvent including an ester moiety include,
but are not limited to, methyl acetate, ethyl acetate, n-propyl acetate,
dimethylacetate, methylpropionate, ethylpropionate,
γ-butyrolactone, decanolide, valerolactone, mevalonolactone,
caprolactone, and the like. Examples of an organic solvent including an
ether moiety include, but are not limited to, dibutylether, tetraglyme,
diglyme, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and
the like. Examples of an organic solvent including a ketone moiety
include, but are not limited to, cyclohexanone and the like. Examples of
an organic solvent including an alcohol moiety include, but are not
limited to, ethyl alcohol, isopropyl alcohol, and the like.

[0048] In some embodiments, the organic solvent may be used singularly or
in a mixture.

[0049] The lithium salt supplies lithium ions in the battery, and performs
a basic operation of a rechargeable lithium battery and improves lithium
ion transport between positive and negative electrodes.

[0051] In some embodiments, the lithium salt may be used at a
concentration ranging from about 0.1 to about 2.0M. Electrolyte
performance and lithium ion mobility may be enhanced due to optimal
electrolyte conductivity and viscosity when the lithium salt a
concentration ranging from about 0.1 to about 2.0M.

[0052] In some embodiments, the positive electrode 114 can include a
current collector and a positive active material layer disposed on the
current collector. In some embodiments, the positive active material
layer can include a positive active material, a binder, and optionally a
conductive material.

[0053] In some embodiments, the current collector may be aluminum (Al),
but is not limited thereto.

[0054] In one embodiment, the positive active material may be
manganese-based oxide.

[0055] The manganese-based oxide may be a compound represented by the
following Chemical Formula 4.

LiaMnbO2 [Chemical Formula 4]

[0056] In Chemical Formula 4, 0.5≦a≦1.5, and
1≦b≦3.

[0057] In some embodiments, the rechargeable lithium battery including the
manganese-based oxide may be used for an electric tool and the like
requiring high power. The manganese-based oxide can increase resistance
during storage at an elevated temperature.

[0058] In some embodiments, fluoroethylene carbonate, lithium
bis(oxalato)borate, and tris(trialkylsilyl)borate can be included in an
electrolyte. In some embodiments, the inclusion of fluoroethylene
carbonate, lithium bis(oxalato)borate, and tris(trialkylsilyl)borate may
prevent resistance increase of a rechargeable lithium battery including
manganese-based oxide when stored at an elevated temperature and thus,
improve stability of the rechargeable lithium battery at an elevated
temperature.

[0059] In some embodiments, the positive active material may include
nickel-based oxide along with the manganese-based oxide.

[0060] In some embodiments, the nickel-based oxide may include a compound
represented by the following Chemical Formula 5, a compound represented
by the following Chemical Formula 6, or a combination thereof.

[0063] In some embodiments, 70 to 90 wt % of the manganese-based oxide may
be mixed with 10 to 30 wt % of the nickel-based oxide. In some
embodiments, 80 to 90 wt % of the manganese-based oxide may be mixed with
10 to 20 wt % of the nickel-based oxide. In some embodiments, the
manganese-based oxide can be used to afford a rechargeable lithium
battery with excellent storability at an elevated temperature due to
resistance decrease when allowed to stand at an elevated temperature.

[0064] In some embodiments, the binder can improve binding properties of
the positive active material particles to each other and to a current
collector. Examples of the binder include, but are not limited to, at
least one component selected from the group consisting of polyvinyl
alcohol, carboxylmethyl cellulose, hydroxypropyl cellulose, diacetyl
cellulose, polyvinylchloride, carboxylated polyvinyl chloride,
polyvinylfluoride, an ethylene oxide-containing polymer,
polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene,
polyvinylidene fluoride, polyethylene, polypropylene, a styrene-butadiene
rubber, an acrylated styrene-butadiene rubber, an epoxy resin, nylon, and
the like.

[0065] In some embodiments, the conductive material can be used in order
to improve conductivity of an electrode. Any electrically conductive
material may be used as a conductive material unless it causes a chemical
change. Examples of the conductive material include, but are not limited
to, natural graphite, artificial graphite, carbon black, acetylene black,
ketjen black, a carbon fiber, a metal powder or a metal fiber including
copper, nickel, aluminum, silver, and so on, and a polyphenylene
derivative.

[0066] In some embodiments, the negative electrode 112 includes a negative
current collector and a negative active material layer disposed on the
negative current collector.

[0067] In some embodiments, the current collector may include a copper
foil.

[0068] In some embodiments, the negative active material layer includes a
negative active material, a binder, and optionally a conductive material.

[0069] In some embodiments, the negative active material includes a
material that reversibly intercalates/deintercalates lithium ions, a
lithium metal, a lithium metal alloy, a material being capable of doping
and dedoping lithium, or a transition metal oxide.

[0070] In some embodiments, the material that reversibly
intercalates/deintercalates lithium ions can include a carbon material.
In some embodiments, the carbon material may be any generally-used
carbon-based negative active material in a lithium ion rechargeable
battery. Examples of the carbon material include, but are not limited to,
crystalline carbon, amorphous carbon, and mixtures thereof. In some
embodiments, the crystalline carbon may be irregularly-shaped, or may be
sheet, flake, spherical, or fiber shaped natural graphite or artificial
graphite. In some embodiments, the amorphous carbon may be a soft carbon,
a hard carbon, mesophase pitch carbonized product, fired coke, and the
like.

[0071] In some embodiments, the lithium metal alloy can include lithium
and a metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be,
Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn.

[0072] Examples of the material being capable of doping and dedoping
lithium include Si, SiOx (0<x<2), a Si--Y alloy (where Y is an
element selected from the group consisting of an alkali metal, an
alkali-earth metal, group 13 to 16 elements, a transition element, a rare
earth element, and combinations thereof, and is not Si), Sn, SnO2, a
Sn--Y alloy (where Y is an element selected from the group consisting of
an alkali metal, an alkali-earth metal, group 13 to 16 elements, a
transition element, a rare earth element, and combinations thereof and is
not Sn), or mixtures thereof. In some embodiments, one or more of these
materials may be mixed with SiO2. In some embodiments, Y may include
one selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti,
Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs,
Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ti, Ge, P, As, Sb,
Bi, S, Se, Te, Po, and a combination thereof.

[0073] Examples of the transition metal oxide include vanadium oxide,
lithium vanadium oxide, and the like.

[0074] In some embodiments, the binder improves binding properties of the
negative active material particles to each other and to a current
collector. Examples of the binder include but are not limited to, at
least one selected from the group consisting of polyvinyl alcohol,
carboxylmethyl cellulose, hydroxypropyl cellulose, polyvinyl chloride,
carboxylated polyvinylchloride, polyvinylfluoride, an ethylene
oxide-containing polymer, polyvinylpyrrolidone, polyurethane,
polytetrafluoroethylene, polyvinylidene fluoride, polyethylene,
polypropylene, a styrene-butadiene rubber, an acrylated styrene-butadiene
rubber, an epoxy resin, nylon, and the like.

[0075] Any electrically conductive material may be used as a conductive
material unless it causes a chemical change. Examples of the conductive
material include: carbon-based materials such as natural graphite,
artificial graphite, carbon black, acetylene black, ketjen black, a
carbon fiber, and the like; a metal-based material of a metal powder or a
metal fiber including copper, nickel, aluminum, silver, and the like; a
conductive polymer such as a polyphenylene derivative; and mixtures
thereof.

[0076] In some embodiments, the negative electrode 112 and the positive
electrode 114 may be manufactured by a method including mixing the active
material, a conductive material, and a binder to provide an active
material composition, and coating the composition on a current collector.

[0077] In some embodiments, the solvent can be N-methylpyrrolidone, but it
is not limited thereto.

[0078] In some embodiments, the separator 113 may be formed as a single
layer or a multilayer, and may be made of polyethylene, polypropylene,
polyvinylidene fluoride, or a combination thereof.

[0079] Hereinafter, examples of one or more embodiments will be described
in more detail including comparative examples. However, these examples
are not intended to limit the scope of the one or more embodiments.

[0080] In the following examples, if the detailed description of the
already known structure and operation may confuse the subject matter of
the present disclosure, the detailed description thereof will be omitted.

Fabrication of Rechargeable Lithium Battery Cell

Examples 1 to 4 and Comparative Examples 1 to 12

[0081] LiPF6 with a concentration of 1.5M was dissolved in a solution
of ethylene carbonate (EC), ethylmethyl carbonate (EMC), and dimethyl
carbonate (DMC) in a volume ratio of 2:2:6. Fluoroethylene carbonate
(FEC), lithium bis(oxalato)borate (LiBOB), and tris(trimethylsilyl)borate
(TMSB) were added thereto as an additive as provided in Table 1,
preparing an electrolyte solution.

[0082] Next, a positive active material layer composition was prepared by
respectively mixing LiMn2O4 as a positive active material,
polyvinylidene fluoride (PVDF) as a binder, and carbon as a conductive
material in a weight ratio of 92:4:4 and dispersing the mixture into
N-methyl-2-pyrrolidone. The positive active material layer composition
was coated on a 20 μm thick aluminum foil and then, dried and
compressed, fabricating a positive electrode.

[0083] The positive electrode, metal lithium as a counter electrode, and
the electrolyte solution were used to fabricate a coin-type half-cell.

Example 5 and Comparative Example 13

[0084] A positive active material layer composition was prepared by mixing
a positive active material including a mixture of LiMn2O4 and
LiNi0.5Co0.2Mn0.3O2 in a weight ratio of 8:2,
polyvinylidene fluoride (PVDF) as a binder, and carbon as a conductive
material in a weight ratio of 92:4:4 and dispersing the mixture into
N-methyl-2-pyrrolidone. The positive active material layer composition
was coated on a 20 μm-thick aluminum foil and then, dried and
compressed, fabricating a positive electrode.

[0085] A coin-type half-cell was fabricated by using the positive
electrode, metal lithium as a counter electrode, and an electrolyte
solution prepared in the same amount as provided in Table 1 according to
the same method as Example 1.

Comparative Examples 14 and 15

[0086] A positive active material layer composition was prepared by mixing
LiCoO2 as a positive active material, polyvinylidene fluoride
(PVDF), and carbon as a conductive material in a weight ratio of 92:4:4
and dispersing the mixture into N-methyl-2-pyrrolidone. The positive
active material layer composition was coated on a 20 μm-thick aluminum
foil and then, dried and compressed, affording a positive electrode.

[0087] The positive electrode was used with metal lithium as a counter
electrode and an electrolyte solution prepared in an amount provided in
Table 1 according to the same method as Example 1 to fabricate a
half-cell.

Evaluation 1: Resistance of Rechargeable Lithium Battery Cells

[0088] The half-cells according to Examples 1 to 5 and Comparative
Examples 1 to 15 were measured regarding DC internal resistance (DC-IR)
when stored at an elevated temperature. The results are provided in Table
1.

[0089] The DC-IR was measured according to the following method.

[0090] The half-cells according to Examples 1 to 5 and Comparative
Examples 1 to 15 were charged at 4 A and 4.2V at a room temperature of
25° C. and cut-off at 100 mA and then, allowed to stand for 30
minutes. Next, the half-cells were respectively discharged at 10 A for 10
seconds, at 1 A for 10 seconds, and at 10 A for 4 seconds and measured
regarding current and voltage at 18 seconds and 23 seconds, respectively
and then, calculated regarding initial resistance (a resistance
difference between 18 seconds and 23 seconds) according to the
ΔR=ΔV/ΔI formula.

[0091] In addition, the half-cells according to Examples 1 to 5 and
Comparative Examples 1 to 15 were allowed to stand at an elevated
temperature of 60° C. for 50 days and measured regarding
resistance at the elevated temperature according to the same method as
the initial resistance.

[0092] DC-IR (mΩ) variation values in Table 1 indicate a difference
between initial resistance and resistance at an elevated temperature.

[0093] Referring to Table 1, when the half-cells including manganese-based
oxide as a positive active material and fluoroethylene carbonate, lithium
bis(oxalato)borate, and tris(trialkylsilyl)borate in an electrolyte
solution according to Examples 1 to 5 were allowed to stand at an
elevated temperature, they had a lower DC-1R increase rate than ones
according to Comparative Examples 1 to 15. Accordingly, the rechargeable
lithium battery cells afforded excellent elevated temperature stability.

[0094] The electrolyte solutions according to Example 1 and Comparative
Example 1 were analyzed regarding movement using LSV (Linear Sweep
Voltammetry). The results are provided in FIG. 2.

[0095] The LSV analysis was performed by allowing the electrolyte
solutions according to Example 1 and Comparative Example 1 to reside at
60° C. in a chamber for 5 days and measuring them at a voltage
ranging from 3V to 7V at a scan speed of 0.1 mV/s. Herein, a platinum
electrode was used as a working electrode, while lithium metals were used
for a reference electrode and a counter electrode.

[0096]FIG. 2 shows LSV(Linear Sweep Voltammetry) graph of the electrolyte
solutions according to Example 1 and Comparative Example 1 when allowed
to stand at a elevated temperature. Referring to FIG. 2, the electrolyte
solution including fluoroethylene carbonate, lithium bis(oxalato)borate,
and tris(trialkylsilyl)borate all according to Example 1 had a higher
current peak at a region of 5V or more than the one including no additive
according to Comparative Example 1. This results shows that the
decomposition of the electrolyte solution including an additive in a
region of 5V or more was started, which forms a coating layer on an
electrode and decreases resistance. Accordingly, a rechargeable lithium
battery cell may afford elevated temperature stability.

[0097] The half-cells according to Example 1 and Comparative Example 1
were measured regarding each impedance when stored at a room temperature
and an elevated temperature to evaluate resistance. The results are
provided in FIG. 3.

[0098] The impedance was measured in the following method.

[0099] The half-cells according to Example 1 and Comparative Example 1
were charged with 4 A and 4.2V at a room temperature of 25° C. and
cut off at 100 mA to measure initial impedance under a very small
excitation amplitude ranging from 5 to 10 mV and a frequency ranging from
1 MHz to 1 mHz.

[0100] In addition, the half-cells according to Example 1 and Comparative
Example 1 were held at an elevated temperature of 70° C. for 20
days and then, measured regarding impedance at an elevated temperature
according to the same method as above.

[0101]FIG. 3 provides a graph showing initial impedance of the
rechargeable lithium battery cells according to Example 1 and Comparative
Example 1 and their impedance when allowed to stand at an elevated
temperature. In FIG. 3, Z'(Ω) of a horizontal axis indicates a real
impedance, while Z''(Ω) in a vertical axis indicates an imaginary
impedance.

[0102] Referring to FIG. 3, the half-cell of Example 1 has a smaller
semicircle of impedance when allowed to stand at an elevated temperature
against its initial impedance. On the other hand, the half-cell of
Comparative Example 1 has a bigger semicircle of impedance when allowed
to stand at an elevated temperature against its initial impedance.
According to one embodiment of the present invention, a rechargeable
lithium battery cell may have decreased resistance and accomplish
excellent stability at an elevated temperature.

[0103] While the present embodiments have been described in connection
with what is presently considered to be practical exemplary embodiments,
it is to be understood that the invention is not limited to the disclosed
embodiments and is intended to cover various modifications and equivalent
arrangements included within the spirit and scope of the appended claims.